diff --git a/lib/CodeGen/InterferenceCache.cpp b/lib/CodeGen/InterferenceCache.cpp
index 427225dbdcd..fed28668cc6 100644
--- a/lib/CodeGen/InterferenceCache.cpp
+++ b/lib/CodeGen/InterferenceCache.cpp
@@ -22,6 +22,22 @@ using namespace llvm;
 // Static member used for null interference cursors.
 InterferenceCache::BlockInterference InterferenceCache::Cursor::NoInterference;
 
+// Initializes PhysRegEntries (instead of a SmallVector, PhysRegEntries is a
+// buffer of size NumPhysRegs to speed up alloc/clear for targets with large
+// reg files). Calloced memory is used for good form, and quites tools like
+// Valgrind too, but zero initialized memory is not required by the algorithm:
+// this is because PhysRegEntries works like a SparseSet and its entries are
+// only valid when there is a corresponding CacheEntries assignment. There is
+// also support for when pass managers are reused for targets with different
+// numbers of PhysRegs: in this case PhysRegEntries is freed and reinitialized.
+void InterferenceCache::reinitPhysRegEntries() {
+  if (PhysRegEntriesCount == TRI->getNumRegs()) return;
+  free(PhysRegEntries);
+  PhysRegEntriesCount = TRI->getNumRegs();
+  PhysRegEntries = (unsigned char*)
+    calloc(PhysRegEntriesCount, sizeof(unsigned char));
+}
+
 void InterferenceCache::init(MachineFunction *mf,
                              LiveIntervalUnion *liuarray,
                              SlotIndexes *indexes,
@@ -30,7 +46,7 @@ void InterferenceCache::init(MachineFunction *mf,
   MF = mf;
   LIUArray = liuarray;
   TRI = tri;
-  PhysRegEntries.assign(TRI->getNumRegs(), 0);
+  reinitPhysRegEntries();
   for (unsigned i = 0; i != CacheEntries; ++i)
     Entries[i].clear(mf, indexes, lis);
 }
diff --git a/lib/CodeGen/InterferenceCache.h b/lib/CodeGen/InterferenceCache.h
index 800f7055752..d3482d0f3a9 100644
--- a/lib/CodeGen/InterferenceCache.h
+++ b/lib/CodeGen/InterferenceCache.h
@@ -135,7 +135,8 @@ class InterferenceCache {
 
   // Point to an entry for each physreg. The entry pointed to may not be up to
   // date, and it may have been reused for a different physreg.
-  SmallVector<unsigned char, 2> PhysRegEntries;
+  unsigned char* PhysRegEntries;
+  size_t PhysRegEntriesCount;
 
   // Next round-robin entry to be picked.
   unsigned RoundRobin;
@@ -147,7 +148,14 @@ class InterferenceCache {
   Entry *get(unsigned PhysReg);
 
 public:
-  InterferenceCache() : TRI(0), LIUArray(0), MF(0), RoundRobin(0) {}
+  InterferenceCache() : TRI(0), LIUArray(0), MF(0), PhysRegEntries(NULL),
+                        PhysRegEntriesCount(0), RoundRobin(0) {}
+
+  ~InterferenceCache() {
+    free(PhysRegEntries);
+  }
+
+  void reinitPhysRegEntries();
 
   /// init - Prepare cache for a new function.
   void init(MachineFunction*, LiveIntervalUnion*, SlotIndexes*, LiveIntervals*,